This invention relates to techniques for producing a variety of clad steels by joining together in laminated state a plurality of metal materials by electron-beam welding to prepare a welded blank and then rolling or otherwise stretching the welded blank into the desired clad steel materials.
More particularly, the invention relates to a method of producing clad steel materials as products in which method a superhigh-output electron beam is directed deeply into and along the interface between intimately contacting blanks of metals to be welded thereby to weld together all parts of the blanks thus in mutual contact at the interface therebetween.
Steel materials such as steel plates, steel pipes, steel bars, and steel shapes are being used widely and in great quantities in various industrial fields for constructing a broad range of structures such as bridges, buildings, piping and vessels, chemical plants, oil-field equipment, pipelines for fluids, and heat exchangers. These materials in most cases are required to have not only mechanical strength but also durability over long periods of use, that is, properties such as heat resistance, corrosion resistance, and wear resistance.
While the material conditions of these steel materials with respect to the material properties, thickness, etc. are determined from the strength aspect as a first consideration, the conditions for the aspect of the above mentioned durability do not coincide with those for strength. That is, a steel of high tensile strength does not necessarily have excellent corrosion resistance. Accordingly, it has been the practice to resort to measures such as cladding, simple lining, and coating with appropriate metal materials, single non-metallic materials, mixtures thereof, and the like for protecting steel materials.
Particularly in recent years there is a trend toward the building of larger and more complicated structures. This trend has given rise to great increases in not only construction costs but also in running cost relating to operation and maintenance for such measures as corrosion proofing, whereby there is an ever increasing need for steel materials having ample corrosion resistance together with strength.
For example, cases wherein steels of high tensile strength are used as reinforcing materials are increasing. However, high-tensile-strength steels, contrary to their excellent strength characteristic, are not necessarily fully satisfactory on the point of corrosion resistance. Accordingly, as a countermeasure, it is possible to use a stainless steel which has excellent corrosion resistance. A stainless steel, however, is somewhat inferior in strength, whereby it has the demerit of giving rise to thick and heavy construction and high costs.
Accordingly, a technique wherein, with the aim of utilizing the advantages of the two steels thereby to make up for their deficiencies, a high-tensile-strength steel is used as the base metal, and a stainless steel is caused to adhere intimately to the base metal thereby to impart their respective strength and corrosion resistance characteristics to the combination has been proposed. In this case, it is highly desirable on the points of economy and strength of materials that the expensive bonded stainless steel be thin and that the strength of the bond between the metals be high.
As a technique for bonding intimately to one metal material a metal of a different kind, the cladding method is commonly used. For carrying out this cladding, there are, for example, the explosive-cladding method, the weld-overlaying method, and the hot-rolling method.
The explosive-cladding method utilizes the application of instantaneous high pressure accompanying the explosion of an explosive. This method requires expensive equipment and, moreover, safety measures and means, whereby it is not easily practiced.
The weld-overlaying method comprises welding and adding a molten padding of a stainless-steel filler material on the surface of a base material of carbon steel. This method is accompanied by the problems such as welding strain imparted by the welding heat during the welding, the long time required for the welding work, low work efficiency, and limitation of the selectable filler materials of good weldability.
The method of producing electric-resistance welded steel pipes from clad steel strip formed into tubes, which is similar to this, also requires much labor and has other drawbacks such as precipitation of carbides due to heat of welding and the necessity of homogenizing the welded structure by head treatment after welding.
The hot-rolling method is a historically old technique in which a metal material is placed intimately against a metal material of another kind after their contact faces have been cleansed, and then the two materials are hot rolled, in which process step, fresh surfaces are generated on the contact faces and the two materials are roll welded while they are still at a high temperature. Accordingly, the provision beforehand of clean contact faces is an indispensable condition. Therefore, in order to satisfy this condition, the presence of residual air in the contact faces and the formation of oxide films thereon must be absolutely avoided. Furthermore, this method requires a large reduction ratio and a high rolling temperature, for which various measures have heretofore been resorted to, but the means have been disadvantageously complicated and inefficient in many instances.
With the aim of overcoming these difficulties, hot-rolling methods wherein joining of the metal materials by electron-beam welding, as disclosed in Japanese Patent Publication No. 19819/1976, are being proposed. In such a hot-rolling method, troublesome work in the joining process such as removal of residual air on the contact face with a vacuum pump, deoxidation, and pretreatment and filling of a denitrification agent are eliminated, and efficient production of clad steel is made possible. However, this method is accompanied by certain difficulties as will be described hereinafter in conjunction with illustrations.